Document and Entity Information Document
9 Months Ended
Sep. 30, 2018
Document and Entity Information [Line Items]  
Document Type 10-Q
Document Period End Date Sep. 30, 2018
Entity Registrant Name ALLETE INC
Entity Filer Category Large Accelerated Filer
Entity Common Stock, Shares Outstanding 51,424,535
Entity Central Index Key 0000066756
Entity Tax Identification Number 410418150
Current Fiscal Year End Date --12-31
Entity Small Business false
Entity Emerging Growth Company false
Entity Current Reporting Status Yes
Document Fiscal Year Focus 2018
Document Fiscal Period Focus Q3
Amendment Flag false